This invention relates to audible alarm devices and is more particularly concerned with a device that detects motion as low frequency vibration and emits an audible high pitch or an ultrasonic tone.
The invention is more specifically directed to a simple, reliable, low-current-drain audible alarm circuit.
It has been desired to produce a simple and reliable alarm circuit which can serve as an intrusion alarm, or an alarm to shoo a cat, dog, or other pet away from furniture such as a chair or table. It has been discovered that vibrations in the range of about 20 Hz to 250 Hz are characteristic of the vibrations induced by a dog or cat walking on, landing on, or scratching a bed, chair, sofa, table, or shelf. However, there are at present no vibration alarms sensitive to low-frequency motion induced vibration. Instead, these conventional alarms are sensitive to high frequency sound, and can false-trigger by speech, hand claps, whistling, or other environmental sounds.
Also, in order to achieve long battery life, the device should have an extremely low current draw, on the order of a few microamps, while awaiting actuation. However, low power draw has been difficult to achieve.
A number of intrusion detectors on vibration detectors with audible alarms have been previously proposed. Several of these have attempted to use the same transducer both as a sensor and also as an audible sounder. Examples of such circuits are described in Morris U.S. Pat. No. 4,170,769; Kaminishi U.S. Pat. No. 4,470,040; and Hyman U.S. Pat. No. 4,974,850. These circuits rely on the resonance of the transducer to enhance detected signal strength of intruder-generated vibration to bring the signal strength to a level where a switching type oscillator can be actuated. This approach has several major deficiencies for the present purposes. First, detection is limited to frequencies around resonance, which is typically several kilohertz, for a cost-effective design. Thus low frequency vibrations, which are most characteristic of intrusion, go undetected. Second, the detector is very sensitive to ambient noises especially at frequencies near resonance. Thus, the system is highly susceptible to false triggering at frequencies near resonance. Third, these switching threshold circuits are quite sensitive to temperature and to supply voltage variation, and thus require additional components to adjust for these variations or to accommodate changes in environmental conditions.